Single-ended signaling approaches transmit information by sending a single-ended electrical signal over a single signal conductor. Differential signaling approaches, by contrast, transmit information by sending two complementary (i.e., differential) signals over two separate signal conductors. Single-ended signaling typically costs less, but lacks the noise rejection benefits of differential signaling. Consequently, both signaling approaches appear in common electrical circuits.
A balun converts single-ended signals to differential signals, and vice versa, and thereby enables circuits that use different signaling approaches to connect to one another (e.g., antennas and differential amplifiers). In one typical implementation of single-ended to differential signal conversion, a balun consists of a symmetric transformer that has a center tap in its secondary winding. When converting to differential signals, the transformer's primary winding receives the single-ended signal and the transformer's secondary winding generates the differential signals relative to a common reference signal present at the center tap.
Ideally, the differential signals generated by a balun are balanced in the sense that they have the same amplitudes, but opposite phases. In practice, however, achieving perfect balance between differential signals proves quite challenging, especially for a wide range of signal frequencies. Unavoidable asymmetry in the physical layout of an on-chip balun, for example, introduces imbalance between the amplitudes, phases, or both of differential signals. Process variation in the manufacture of on-chip balun components for semi-conductor chips also introduces imbalance. Further, parasitic cross-coupling capacitance between the primary winding and the second winding introduces imbalance when converting signals with high frequency.
Imbalanced differential signals significantly limit performance of wireless communication systems. As one example, the transmitter of a radio front-end employing frequency-division duplexing (FDD) sometimes produces a large amplitude leakage into the receiver. This transmit signal leakage in turn causes intermodulation distortions in the receiver, including for example second inter-modulation distortions known as IMD2. If the receiver directly down-converts the received radio signal to baseband, without first translating the received signal to an intermediate frequency, the second-order inter-modulation distortions disturb the baseband signal. The receiver can theoretically employ a balun to produce differential radio frequency signals and use differential circuits to cancel such even order distortions before they disturb the baseband signal. However, without well balanced differential signals the differential circuits cannot completely eliminate the distortions.